Monolithic three-dimensional structures

ABSTRACT

Three-dimensional structures of arbitrary shape are fabricated on the surface of a substrate through a series of processing steps wherein a monolithic structure is fabricated in successive layers. A first layer of photoresist material is spun onto a substrate surface and is exposed in a desired pattern corresponding to the shape of a final structure, at a corresponding cross-sectional level in the structure. The layer is not developed after exposure; instead, a second layer of photoresist material is deposited and is also exposed in a desired pattern. Subsequent layers are spun onto the top surface of prior layers and exposed, and upon completion of the succession of layers each defining corresponding levels of the desired structure, the layers are all developed at the same time leaving the three-dimensional structure.

BACKGROUND OF THE INVENTION

[0001] The present invention relates, in general, to methods forfabricating monolithic three-dimensional structures on a substrate, andmore particularly to methods for fabricating optical couplers forintegrated laser and waveguide structures.

[0002] Advances in the technology available for processing andfabricating semiconductor devices have allowed structures of variousshapes to be formed on the surface of a wafer, as by the use of avariety of photosensitive materials applied to the surface of thesubstrate and various photolithographic processes for definingstructures to be fabricated. For example, conventional photoresistmaterials can be spun onto a substrate surface and then exposed to lightin specified regions, as through the use of photolithographic masks, tocreate patterns on the substrate after the photoresist has beendeveloped. Such techniques may be used, for example, to fabricateintegrated lasers and waveguides, including ring lasers with a varietyof cavity configurations such as those described in U.S. Pat. No.5,132,983 and in copending U.S. patent application Ser. No. 09/918,544,filed Aug. 1, 2001, the disclosures of which are hereby incorporatedherein by reference. The development of these technologies and thecapability of producing a wide range of laser and waveguide structuresexpands the prospective applications for integrated optical devices, andadds the attractiveness of greater manufacturability and reduced cost.

[0003] Optical couplers are conventionally used to couple light to andfrom integrated optical laser and waveguide devices; however, easy andefficient techniques for coupling such devices with external componentssuch as optical fibers are not available. Although optical couplers ofvarious designs have been developed, there is great difficulty inaligning such couplers with integrated optical devices and with externaloptical components such as optical fibers, and the resulting low yieldproduces high costs for such devices. Accordingly, a monolithic opticalcoupler having an arbitrary three-dimensional pattern would be verydesirable, for it would enable cost effective, high yield fabrication ofintegrated optical components and their couplers to enable coupling ofthe devices with external components such as optical fibers.

SUMMARY OF THE INVENTION

[0004] In accordance with the present invention, three-dimensionalstructures of arbitrary shape are fabricated on the surface of asubstrate through a series of processing steps which form a monolithicstructure by fabricating it in successive layers. These layerspreferably are formed from a lithographically definable material such asconducting polymers, resist materials, or the like. For convenience, thefollowing description will refer to layers formed of photoresistmaterials, but it will be understood that such other materials can alsobe used. Thus, for example, in accordance with a preferred form of theinvention, a first layer of a photoresist material is spun onto asubstrate surface and is exposed to a desired pattern corresponding tothe shape of the final structure at a corresponding level in thestructure. The layer film is not developed after exposure; instead asecond layer of photoresist material is deposited on top of the firstlayer and is also exposed to a pattern at least partially verticallyaligned with the first pattern, and corresponding level of this layer.Subsequent layers are spun onto the top surface of the prior layer andadditional aligned patterns are exposed. If desired, a barrier layer maybe provided between successive layers to prevent intermixing. Uponcompletion of the successive vertically aligned layers defining thestructures, the layers are all developed at the same time. This removesthe exposed material (in the case of a positive photoresist) leaving theunexposed material behind to form a three-dimensional structure havinglevels corresponding to the exposed patterns. In the case of a negativephotoresist material, the exposed material forms the structure.

[0005] The foregoing process utilizes either a positive or negativephotoresist material, but in a modification of the process, thethree-dimensional structure can be fabricated using some positivephotosensitive materials and some negative photosensitive materials. Theexposure of the photoresist materials preferably is donephotolithographically, allowing a wide range of shapes andconfigurations. It will be understood, however, that e-beam, x-ray orother forms of radiation may be used to expose corresponding resist orother lithographically definable layers using the same layeringtechnique disclosed herein.

[0006] The fabrication techniques of the invention may be used toproduce optical couplers, gratings, and other multilayer devices ofarbitrary shape for use with optical systems, in integrated circuitsystems, and the like, where multilayered structures are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The foregoing, and additional objects, features and advantages ofthe present invention will be apparent to those of skill in the art fromthe following detailed description of preferred embodiments thereof,taken in conjunction with the accompanying drawings, in which:

[0008] FIGS. 1(a) through 1(f) illustrate the process steps used in theformation of a monolithic three-dimensional structure in accordance witha first embodiment of the present invention:

[0009]FIG. 2 is a diagrammatic illustration of a monolithicthree-dimensional structure fabricated in accordance with the steps ofFIG. 1;

[0010] FIGS. 3(a) through 3(f) illustrate the process steps for forminga monolithic three-dimensional structure in accordance with a secondembodiment of the invention;

[0011] FIGS. 4(a) through 4(d) illustrate a process for fabricating agrating utilizing the process steps of the present invention, inaccordance with a third embodiment of the invention; and

[0012]FIG. 5 is a diagrammatic illustration of three-dimensionalstructures in accordance with the present invention for coupling opticalelements to optical fibers.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] Turning now to a more detailed description of the presentinvention, FIGS. 1(a)-1(f) illustrate a series of processing steps whichare used to form a monolithic structure in accordance with the presentinvention. As will be described in greater detail, a solidthree-dimensional structure of an arbitrary shape is formed on thesurface of a substrate by fabricating a series of successive layers,which may be of different thickness, and processing each layer in turn.These layers correspond to cross sectional levels in the desiredarbitrary three-dimensional structure so that the entire thickness ofthe structure is built up through the use of successively exposed layersof photosensitive material. These successive layers are illustrated inFIGS. 1(a)-1(e), to which reference is now made. In accordance with theprocess of the invention, a substrate 10, illustrated in cross-sectionalview 12, carries a first layer 14 of a conventional resist material, asillustrated in top plan view at 16. This top layer 14 may, for example,be a photoresist layer that has been spun on the top surface 18 of thesubstrate, and may be of any desired thickness. This thicknesscorresponds to the desired height of a first level 20 of athree-dimensional structure 22 to be formed on the surface of thesubstrate, an example of such a structure being illustrated in FIG. 1(f)and in FIG. 2. Conventional photolithographic tools, such as aprojection aligner, are used to expose the layer 14 of the photoresistfilm through a pattern in the desired regions of the film, in accordancewith the arbitrary shape of the structure to be fabricated. In theillustrated embodiment, such a pattern includes a first portion 24 ofthe resist layer 14 which is masked so that it remains unexposed, withthe remaining portion 26 of layer 14 being exposed to a suitable sourceof light, in known manner. The unexposed portion 24 corresponds in shapeand thickness to the first level 20 of the structure 22.

[0014] Unlike conventional lithography, the resist layer 14 is notdeveloped after the exposure; instead, a second layer 30, which also maybe a conventional photoresist material, is spun onto the top surface ofthe first layer 14. Depending on the type of thelithographically-definable material used for the second layer 30, theremay be a need for a barrier film between layers 14 and 30, to preventintermixing of these two layers (see EXAMPLE below). Thereafter, thesecond layer 30 is masked and exposed at 32, leaving a second unexposedregion 34 which corresponds in thickness and in shape to thecross-sectional level 26 of the three-dimensional structure 22, alsoillustrated in FIGS. 1(f) and 2.

[0015] It should be noted that the exposure time is selected to be justsufficient to penetrate the second photoresist layer 30, butinsufficient to penetrate into the first photoresist layer 14. This isreadily accomplished, since many photoresists have higher absorptionwhen they are unexposed as compared to when they are exposed. However, aslight exposure into the first photoresist layer 14 can be tolerated.The photoresist material is absorbing at the exposure wavelength,thereby helping to prevent light from reaching the lower layers,although it is more transparent at the wavelength of the optical devicewith which it is to be used.

[0016] After exposure of the second layer 30, and before development ofthe exposed material, a third layer 40 of a conventional photoresistmaterial is applied to the top surface of second layer 30. Again, abarrier film may be applied, if needed, and the thickness of the thirdlayer is selected to correspond to the desired thickness of the thirdlevel 42 of the three-dimensional structure 22, as illustrated in FIG.1(f) and FIG. 2. The third layer is exposed, as before, in region 44,leaving an unexposed region 46 corresponding to level 42, as illustratedin the cross-sectional view 48 and in the top view 50.

[0017] In the illustrated embodiment, a fourth layer 52 is applied tothe top surface of layer 40 and is exposed in region 54, leaving anunexposed region 56. Again, the photoresist layer 52 is exposedphotolithographically through a suitable mask for a time period which issufficient to expose layer 52 but not layer 40. The thickness of layer52, illustrated in the cross-sectional view 58, corresponds to thethickness of level 60, illustrated in FIG. 1(f) and FIG. 2, with itslength and width dimensions, illustrated in the top view 62, being ofany desired shape.

[0018] Finally, in the illustrated embodiment, a fifth layer 64 isapplied to the top surface of layer 52 and is exposed at region 66, asillustrated in the cross section 68 and the top view 70. The top layer64 is masked to leave a selected region 72 unexposed, with this regioncorresponding to level 74 of the structure 22, as previously described.The length and width of region 72 is of any selected arbitrary shape, aspreviously discussed. Barrier films may be provided between successivelayers, as described above.

[0019] As a final step, all five layers 14, 30, 40, 52 and 64 of thephotoresist material are developed in a single step, in conventionalmanner, removing the exposed regions 26, 32, 44, 54, and 66 and leavingthe structure 22. Although the various levels of structure 22 areillustrated as being generally rectangular, it will be understood thateach level may be of any desired shape or size and may be positioned atany desired location on the substrate.

[0020] An alternative process for fabricating the solidthree-dimensional structure of FIG. 2 is illustrated is FIG. 3, to whichreference is now made. In this process, two different types ofphotoresistive material are used to enable the structure to befabricated in successive exposure steps and in just two layers ofphotoresist material.

[0021] In FIG. 3(a), a substrate 90 is provided with a first layer 92 ofa negative photoresist material, in which sections that are unexposedare removed through development. After the exposure steps to bedescribed, a second layer 94 of photoresist material is applied on thetop surface of layer 92, but this layer is of opposite polarity; i.e.,is positive; that is, sections that have been exposed are removed bydevelopment.

[0022] In the first step, following deposition of layer 92 on substrate90, a first region 96 of layer 92 is exposed through a suitable mask,using conventional photolithography. As illustrated in thecross-sectional view 100 and in the corresponding top view 102, theunexposed region 96 may be of a selected thickness and shape tocorrespond to a first level 104 of a three-dimensional structure 106illustrated in cross section 108 and top view 110 of FIG. 3(f). Theregion 112 surrounding the exposed area 96 remains unexposed in thisnegative photoresist layer.

[0023] In step 2, illustrated in cross section 114 and top view 116 ofFIG. 3(b), the layer 92 is further exposed in conventional manner as bya projection aligner, for a length of time which causes the unexposedmaterial in layer 92 which lies directly below the previously exposedregion 96, now to be exposed through the thickness of layer 92, thuscausing the exposed region 96 to be transferred through the layer 92 toa depth corresponding to level 104 of the structure 106. During thissecond exposure, a second level 120 may be exposed, with level 120 beingof a different size and shape than the region 96. This second exposedarea 120 is at least as large as, and extends to the edges of, region 96in order to transfer region 96 to the lower level of layer 92. Region112′ is masked during this second exposure to remain unexposed.

[0024] After the second exposure has been completed, a photoresist layer94 is deposited on the top surface 122 of layer 92. This layer 94, aspreviously discussed, is of an opposite photosensitivity type than layer92; in this case it is positive. As illustrated in the cross section 130and the top view 132 of FIG. 3(c), the top layer 94 is exposed in region134 through a suitable photolithographic mask, leaving region 136unexposed. The unexposed region 136 corresponds in thickness and inshape (i.e., length and width dimensions) to level 138 of structure 106,the length of exposure of region 134 controlling the thickness of region136.

[0025] Thereafter, as illustrated in cross section at 140 and in topview at 142 in FIG. 3(d), layer 94 is exposed again to extend theoriginal exposure region 134 more deeply into layer 94 and to create anew exposed region 144 which defines a new unexposed region 146corresponding to level 148 of structure 106. Region 146 lies within theboundaries of region 136, and the exposure is timed to produce thedesired thickness of region 144.

[0026] The fifth exposure is illustrated in cross section at 150 and intop view at 152 in FIG. 3(e), where the layer 94 is again exposed aftermasking the region 154, which is to correspond to level 156 in structure106. This produces a new level 158 of exposed resist and forms thelevels 136 and 146 of the layer 94 by further exposure through thepreviously exposed regions 134 and 144 and by initial exposure of theregion 158.

[0027] As a final step, both of the resist layers 92 and 94 aredeveloped, removing the unexposed material from layer 92 and the exposedmaterial from layer 94 and producing the resultant structure 106. Again,although the levels 104, 120, 138, 148 and 156 are all illustrated asbeing rectangular, it will be apparent that any arbitrary shapes may beproduced using this process, as long as each successive layer is shapedand positioned to permit the repeated exposure of previously exposedregions.

[0028] FIGS. 4(a)-4(d) illustrate process steps which may be used tofabricate a multilayered structure in which layers are sequentiallyexposed, and in which all of the layers are developed as a final step,with the development process removing portions of intermediate layers toproduce enclosed openings or channels. The illustrated process is shownas producing an optical grating, but it will be apparent that otherarbitrary shapes and configurations can be fabricated. In FIG. 4(a), alayer 170 of photosensitive material is applied to the surface 172 of asubstrate 174, as illustrated in cross section at 176 and in top view at178. As before, this layer can be a photoresist layer that has been spunon the substrate surface with the appropriate thickness. This firstlayer is left unexposed and a second layer 180 of photoresist isdeposited on top of the first layer. Thereafter, a projection aligner isused to expose photoresist layer 180 in spaced linear regions 182, asillustrated in cross section at 184 and in top view at 186. These spacedregions 182 are shown as being linear and closely spaced to form aperiodic optical grating or the like but it will be understood thatother configurations can be provided. The exposure time is chosen sothat the exposed region ends very near the bottom surface of the secondlayer 180, but does not enter substantially into the bottom layer 170.

[0029] Thereafter, a third layer 190 is deposited on the top surface ofthe exposed layer 180 before the second layer is developed, asillustrated in cross section at 192 and in top view at 194 in FIG. 4(c),so as to enclose the structure defined on the second layer.

[0030] Thereafter, a suitable developer is used to remove any parts ofthe photoresist materials that have been exposed, such as the enclosedregions 182 of FIG. 4(b). To accomplish this, the developer must travellaterally underneath the third photoresist layer 190, thereby formingchannels 200 through the second layer and between unexposed segments ofthe second layer. The development time is selected to ensure completeremoval of exposed photoresist, leaving spaced linear segments 202 ofresist material, spaced by channel or cavity regions 200, to therebyform a covered optical grating. Other structures having covered cavitiesor channels may similarly be fabricated using this process.

[0031] The grating described above is a one-dimensional photoniccrystal, however, two- and three-dimensional photonic crystals can alsobe fabricated using the process described hereinabove.

[0032] As illustrated diagrammatically in FIG. 5, the three-dimensionalstructure such as the structure 22 of FIG. 2 can be fabricated on anoptical chip 210 of conventional design and carrying monolithic opticalcomponents such as monolithic lasers or waveguides 212 which have beenpreviously fabricated on the chip 210. These optical components 212 maybe conventional, and may, for example, be waveguides leading to opticaldevices such as the ring lasers described above, or may be lasers. Thethree-dimensional structures 22 may be shaped to match the opticalcomponents 212 with corresponding external optical devices such asoptical fibers 214. The optical couplers 22 are formed through theprocesses described above and are positioned very close to themonolithic optical components 212 on the optical chip. The couplers aredesigned to provide very high efficiency and high positional tolerancefor coupling the optical components to the optical elements 214.

[0033] Although the above-described three-dimensional structure maypreferably be used as an optical coupler in the manner described in FIG.5, it will be apparent such devices may be shaped to serve aswaveguides, and may provide curved as well as linear structures andconnections, since the process described hereinabove may be used tofabricate structures of arbitrary shape. Further, it will be understoodthat the optical couplers not only can be used to connect to externaldevices in the plane of the chip, but with three-dimensional opticalcouplers such external optical devices can be positioned within asemispherical volume above the chip. A transparent chip substrate orappropriate holes within the substrate would allow the external opticaldevices to be positioned within the semispherical volume beneath thechip. It will be further understood that although the above descriptionused photoresist to illustrate the process, any photosensitive materialor other materials that lend themselves to lithographic definition canbe utilized.

EXAMPLE

[0034] Arch Chemicals, Inc. manufactures a photoresist referred to asOIR897-12I on a substrate at 4000 rpm for 30 seconds which results in alayer thickness of 1.2 microns. If layer 30 is also to be formed usingthe same photoresist, it will be necessary to create a barrier filmbetween layers 14 and 30; otherwise, the spinning on of the second layerof photoresist will dissolve layer 14. Shin-Etsu MicroSi manufactures achemical referred to as CEM365IS. This chemical has been used tosuccessfully create a barrier film. The process for forming a simple twolayer structure with a barrier layer is as follows; with reference tothe process of FIG. 1:

[0035] (a) Form layer 14 by spinning on OIR897-12I at 4000 rpm for 30seconds.

[0036] (b) Deposit CEM365IS on top of layer 14.

[0037] (c) Bake sample at 90° C. for 1.5 minutes on a hot-plate (this isthe pre-exposure bake, but also serves to create the barrier film).

[0038] (d) Remove excess CEM365IS from above the barrier film by washingin deionized water.

[0039] (e) Expose layer 14 in desired regions.

[0040] (f) Form layer 30 by spinning on OIR897-12I at 4000 rpm for 30seconds.

[0041] (g) Bake sample at 90° C. for 1.5 minutes on a hot-plate.

[0042] (h) Expose layer 30 in desired regions.

[0043] (i) Develop layers 14 and 30 in one step.

[0044] It will be apparent that this process for producing a barrierlayer between resist layers can be extended for each subsequent layer inthe process described hereinabove.

[0045] Although the present invention has been described in terms ofpreferred embodiments, it will be apparent that innovative modificationsand variations may be made without departing from the true spirit andscope thereof, as set forth in the following claims:

What is claimed is:
 1. A method for fabricating three-dimensionalstructures, comprising: forming a first resist layer on a substrate;exposing a first pattern on said first layer; forming a second resistlayer on top of said first layer; exposing a second pattern on saidfirst layer in at least partial vertical alignment with the firstpattern; and thereafter developing said first and second layers toproduce a structure having levels corresponding to said exposedpatterns.
 2. The method of claim 1, further including successivelyforming multiple additional resist layers on preceding exposed layersand individually exposing each additional layer, and thereafterdeveloping all of said resist layers.
 3. The method of claim 1, furtherincluding: exposing said first layer with plural vertically alignedpatterns prior to forming said second resist layer.
 4. The method ofclaim 3, further including: exposing said second layer with pluralvertically aligned patterns prior to developing said layers.
 5. Themethod of claim 1, further including: exposing said second layer withplural vertically aligned patterns prior to developing said layers.
 6. Amethod for fabricating three-dimensional structures, comprising: forminga first resist layer on a substrate; forming a second resist layer ontop of said first resist layer; exposing a first pattern on said secondresist layer; forming a third resist layer on top of said second layer;and developing the exposed pattern in said second layer to produceenclosed channels extending between said first and third layers.
 7. Amethod for fabricating three-dimensional structures, comprising: formingat least a first lithographically-definable layer on a substrate;performing a first lithographical definition of said first layer;forming at least a second lithographically-definable layer on said firstlayer; performing a second lithographical definition of said secondlayer; said second lithographic definition overlapping at least in someregions with said first lithographic definition; developing said firstand second layers.
 8. The method of claim 7, further including: formingon said first layer a first barrier layer before performing said firstlithographical definition of said first layer.
 9. The method of claim 7,further including: forming barrier layers on each of saidlithographically-definable layers.
 10. A three-dimensional structure,comprising: a first lithographically-defined layer; a secondlithographically-defined layer on top of said first layer; said secondlayer being mechanically supported by first layer.
 11. Thethree-dimensional structure of claim 10, including: a barrier filmpositioned at the interface between said first and second layers.